usb_cdc.serials[1].connected always return False for Raspberry Pi Pico

[ismael-miguel]

Firmware

Adafruit CircuitPython 6.2.0-beta.3 on 2021-03-04; Raspberry Pi Pico with rp2040

Code/REPL

import usb_cdc
usb_cdc.serials[1].connected

Behavior

Running the following code ALWAYS returns False, even when reading data to/from the Raspberry Pi Pico, on that USB CDC channel.

>>> import usb_cdc
>>> usb_cdc.serials[1].connected
False

Additional Info

Despite returning False, the link is working properly.

I can send and receive data, without any issues.

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According to this: https://circuitpython.readthedocs.io/en/latest/shared-bindings/usb_cdc/index.html

True if this Serial is connected to a host. (read-only)

Since it is connected to a host, should return True.

And no, the Raspberry Pi Pico is the only device connected to serial, and both ports belong to it:

[dhalbert]

I do not see this problem with beta.3, either on Windows or Linux. I do have to be connected to serials[1] before it turns to True. In the screenshot below, I at first did not have a connection to COM18, but then when I did connect via Tera Term, you can see it changed from False to True.

.connected means that the COM port has been opened, not just that is is present.

[ismael-miguel]

Interesting ...

When I use C# to connect to it, it returns False, but I can read and send data without issues:

But when I use PuTTY, it return True:

[dhalbert]

I wonder if the C# code you are using is closing the connection after each write or something like that. If you do a read with a timeout in C# so it hangs while you check, does it show as connected during that time?

[ismael-miguel]

The C# code uses the SerialPort.DataReceived event, and waits for a text line.
(https://docs.microsoft.com/en-us/dotnet/api/system.io.ports.serialport.datareceived?view=netframework-4.7.2)

It opens the connection and waits.
There's nothing written to close the connection.

---

I've tried to stop on the serial.ReadLine(); method, in C#:

But it still returns False:

[hathach]

You need to set line state DTR to 1, it is used by tinyusb to detect terminal connection

[dhalbert]

Thanks @hathach. Some websearching shows this is a convention, and that terminal programs typically assert it when opening the conenction. So does pyserial, by default. This does not mean that DTR/DSR-style flow control is enabled. There is no other easy way to determine that something is connected on the host end. I can add more docmentation to .connected to make this clear.

Some more websearching shows that other people have had this problem with C#, which defaults to DTR not being set.

[ismael-miguel]

Thank you @hathach and @dhalbert.

I can confirm that setting serial.DtrEnable = true; in C# solved the issue:

Would be amazing if this was documented, to avoid possible duplicated issues.